1. Field of the Invention
The present invention relates to a method, system, and program for synchronization and resynchronization of a data stream.
2. Description of the Related Art
In data recording systems, a data drive writes positive and negative “flux transitions” to a storage medium using encoding schemes known in the art, and a decoder is used to read back the encoded data stored on the storage medium. A “one” bit (“1”) represents a peak or trough in the signal while a “zero” bit (“0”) indicates no peak. Data to be provided to a recording channel or decoder is a sequential string of bits, to be recorded or readback on/from the recording media as transitions. In the prior art, a data set will include a VFO (“Variable Frequency Oscillator) pattern at a specific location (a header) and a particular known transition frequency to synchronize a read clock (such as a phase locked loop (“PLL”)) to the codeword bit frequency. The VFO pattern may comprise repetitive codeword pattern, such as a sequence of “10” or 2T bits, but which is identifiable because of its location in a header. A known synchronization mark is also provided between the VFO pattern and any encoded data to allow the decoder to align to the codeword boundaries and to the start location of the incoming data.
If the PLL does not achieve complete phase lock, or if the decoder does not align to the codeword boundaries on the incoming data, then the decoder will not be able to successfully decode incoming encoded data until the next synchronization (“sync”) or resynchronization (“resync”) pattern is encountered, usually in the header of the next data set or in a header for a grouping of codeword groups. When a synchronization pattern is not detected (missed) or detected erroneously in the wrong position (e.g., due to a defect), the exact bit position and alignment to codeword boundaries is not known. Accordingly, the decoder will not be able to successfully decode the encoded data unless or until some further sync field is encountered, such as the next sync or resynchronization. Such a failure to decode may result in infinite error propagation, which is a catastrophic type of failure.
Even though the location of the VFO pattern and the synchronization mark are known, misdetection of one or more bits in the synchronization mark may prevent recognition of the synchronization mark. Without proper recognition of the synchronization mark, the data detector will not be in-sync with the recorded data and will not recognize the data.
In the prior art, data errors are addressed using error correcting codes. Such codes allow detection and correction of many data recording errors. However, errors in the VFO or synchronizing patterns may result in missing detection of the synchronization mark or detecting it in the wrong place. Therefore, the data detection may begin with reading data before or after the data really starts. If the start of data is not determined via detection of the synchronization mark, all data between one synchronization mark and a subsequent synchronization mark which is detected successfully will be lost. This may have a catastrophic effect, rendering useless the error correcting codes protecting the data.
In the commonly assigned U.S. Pat. No. 5,999,110, a method is disclosed based on using an encoded synchronization mark concatenated with a VFO pattern wherein the encoded synchronization mark is set at a maximum Hamming distance from the concatenated VFO for the number of bits in the fixed plurality of bits. However, in certain applications, such as an Enhanced Partial Response Type 4 (EPR4) waveform, the maximum Hamming distance measurements are less effective because errors tend to propagate between peaks.
Thus, there is a need in the art to provide more sophisticated synchronization and resynchronization methods to reduce errors in data encoding systems.